EPSC Abstracts Vol. 13, EPSC-DPS2019-891-1, 2019 EPSC-DPS Joint Meeting 2019 c Author(s) 2019. CC Attribution 4.0 license.

LaRa (Lander Radioscience) on the ExoMars 2020 Surface Platform.

Véronique Dehant1,2, Sébastien Le Maistre1, Rose-Marie Baland1, Özgür Karatekin1, Marie-Julie Péters1, Attilio Rivoldini1, Tim Van Hoolst1, Bart Van Hove1, Marie Yseboodt1, and Alexander Kosov3 (1) Royal Observatory of Belgium (ROB), Brussels, Belgium, (2) Université catholique de Louvain, Louvain-la-Neuve, Belgium, (3) Space Research Institute Russian Academy of Sciences (IKI), Moscow, Russia

Abstract The surface platform Kazachok will house the radio science experiment LaRa (Lander Radioscience) to support specific scientific objectives of the ExoMars 2020 mission. The LaRa experiment is described and the scientific objectives and strategies are detailed in this presentation. Figure 1. LaRa principle. 1. Introduction 2. The LaRa instrument In the last decade, several missions and observations As LaRa performs a coherent retransmission of the have brought new data on the planet Mars, obtained uplink carrier to the downlink carrier, the downlink by either spacecraft orbiting around Mars or landers frequency is scaled by a constant multiplier, the and rovers on the surface. Among other things, the transponder ratio (880/749). By emitting the uplink information acquired will improve our understanding signal from Earth, the masers in the ground stations of Mars’ interior, evolution and habitability. Data ensure frequency stability of LaRa’s radiosignal. obtained by new space missions are driving further The LaRa instrument is built in collaboration with progress in this field. In 2020, the European Space other Belgian actors under the lead of ESA PRODEX, Agency ESA and the Russian space agency and ROB providing the instrument specifications. It Roscosmos will launch the ExoMars 2020 mission. It is composed of three antennas (one for receiving the consists of a rover and a surface platform, and will signal from Earth and two for transmitting back to land in the Northern hemisphere of Mars. Belgium Earth) designed by UCLouvain (see Figure 2), and a contributes one of the scientific instruments on the coherent transponder (see Figure 3) designed by platform: a radio transponder called LaRa for Lander Antwerp Space. Radioscience. The LaRa experiment is designed to obtain coherent two-way Doppler measurements from the radio link between the 2020 ExoMars platform and the Earth, during at least one Martian year (see Figure 1). The Doppler measurements will be used to infer the relative radial velocity of the Earth and the lander, Figure 2. Flight Models Figure 3. Flight Model allowing to observe the variations in orientation and of the LaRa Transmitting of the transponder while rotation of Mars in space (i.e. precession, nutations, and receiving antennas. mounting the PCBs and length-of-day variations) as well as polar motion. Flight Models. (Printed Circuit Board). The ultimate objective is to obtain information on the Martian interior and on the sublimation/condensation In 2018, various models of the instrument parts were cycle of atmospheric CO2. Rotational variations will developed: a Thermal Model (TM), Structural Model allow us to constrain the moment of inertia of the (SM), Engineering Model (EM), the Flight Model whole planet, including its mantle and core, as well (FM). The EM, built to test LaRa’s functionalities, as the moment of inertia of the core, and seasonal has been tested at ESOC (European Space Operations mass transfer between the atmosphere and ice caps. Centre, Darmstadt) and has proven its coherence to be better than requirements (Allan deviation of less than 10-13 at 60 seconds). The transponder design maintains the coherency of the signal, and the global precision on the Doppler is expected to be better than 0.05 mm/s with 60 second integration time. By comparison, the requirement for instrument precision was 0.02 mm/s with the same integration time. Figure 4. Representation of precession and During operations, the transponder will function nutation of Mars. when an Earth ground station is available and when the Earth is in the sky of the lander. The position of Additional rotation changes are due to exchange of the lander will be determined from the first passes, as angular momentum with the atmosphere. The well as from the landing site characteristics and rotation rate of Mars is approximately uniform, but MOLA altimetric data [5]. LaRa will operate at least variations in the length-of-day (LOD) have been twice per week during the whole mission lifetime observed and are mainly due to exchanges of mass (twice per week in the minimum guaranteed mission and angular momentum between the atmosphere and and during the extended mission, with a possible surface. These exchanges occur mostly at seasonal relaxation to once per week during hibernation). periods as a result of sublimation/condensation of the CO2 polar ice caps, mass redistribution in the 3. The LaRa science atmosphere, and seasonally changing zonal winds. Since Mars is oblate and rotating, and the equator is LaRa will improve current estimates of the LOD not parallel to the Mars-Sun line (the obliquity angle variations (known to within about 15% [4]), is about 25°), the planet reacts as a spinning top to providing the most accurate constraints on the the gravitational torque exerted by the Sun. As a distribution of atmospheric mass, angular momentum, consequence, the rotation axis and the planet slowly and the ice caps. move in space around the perpendicular to the orbital plane (see Figure 4). The time needed to perform one Acknowledgements cycle around the orbit normal is presently about This work was financially supported by the Belgian 171,000 years with a speed on the precession cone PRODEX program managed by the European Space (so-called precession rate) of about 7.6 arcs/year. A Agency in collaboration with the Belgian Federal first objective of LaRa is to accurately determine the Science Policy Office. precession rate. Because the precession is inversely proportional to the polar principal moment of inertia, References LaRa will be able to determine Mars moments of [1] Dehant, V., and Van Hoolst T. Encyclopedia of the Solar inertia, which provide important constraints on the System, Chapter 8, 159-184, 2014. interior structure. [2] Dehant V. and Mathews P.M., Precession, Nutation, and Wobble of the Earth. Book, Cambridge University Press, ISBN: Because of the elliptical orbit of Mars and the orbital 9781107092549, 536 pages, 2015. changes due to for instance interaction with other [3] Dehant V. and Mathews P.M., Treatise on Geophysics, Vol. 3 Solar System bodies, the gravitational torque on Geodesy, Section 3.10, ISBN: 9780444538024, 2015. Mars changes with time. The variations in the torque [4] Konopliv A.S., Park R.S., Folkner W.M., Icarus, 274(253), induce periodic changes in the precession as well as DOI: 10.1016/j.icarus.2016.02.052, 2016. variations in the obliquity angle, called nutations. [5] Le Maistre S., InSight coordinates determination from direct- The resulting motion of the pole due to precession to-Earth radio-tracking and Mars topography model. Planetary and Space Science, 121, 1-9, 2016. and nutation is wiggly as illustrated in Figure 4. The [6] Van Hoolst T., and Rivoldini A., Encyclopedia of the Solar periods of the nutations are related to the periods of System, Chapter 18, 379-396, 2014. the orbital motion and the orbital perturbations. The [7] Van Hoolst T., Treatise on Geophysics, Vol. 10 Planets and largest of these nutations has a period of half the Moons, Section 10.04, ISBN: 9780444538024, 2015. orbital period ([1]-[3] [6] [7]). Observations of these nutations will lead to constraints on the physical state of the core (confirmation that the core is at least partially liquid), its density, shape, and size.